This invention relates generally to the production of filamentary material and more particularly to a novel spray spinning nozzle for spinning molten polymers to form a nonwoven structure.
Various apparatus has been developed in the past to create an integrated system for forming a fibrous assembly, such as a nonwoven fabric or the like, directly from a molten filament forming material. Typically, such an apparatus may use an extruder in which one of various kinds of synthetic resinous polymeric material is melted under the influence of heat and pressure to form a quantity of molten material which can then be forced through a nozzle orifice as a continuous liquid filament. Each of a plurality of high velocity gaseous jets is directed along the freshly extruded filament at a shallow angle to create a drag force for attenuating the filament which is then carried along by the attenuating aqueous jets and deposited on a collection surface to form a nonwoven structure. Such a device in the past has been known as a spray spinning apparatus because the filamentary material appears to be sprayed against the collection surface.
The attenuating gaseous jets contribute to filament cooling as well as attenuating and conveying the filament to the collection surface. Since the filament of polymeric material is still in a somewhat molten or tacky stage as it strikes the collection surface, some sticking together occurs at each point where the filament contacts itself. Also, the filament may loop about and stick to itself.
One such spray spinning apparatus is shown in U.S. Pat. No. 3,849,040 which is assigned to the assignee of the present patent application. This patent shows a stream of filamentary material emanating from a nozzle. A pair of elongated attenuating gas jets, each with a rectangular cross section, are placed on either side of the nozzle. The gas jet outlets are both in the same plane perpendicular to the nozzle axis, positioned forward of the nozzle orifice and the jets intersect at a point offset from the nozzle axis in the plane of the nozzle axis. The axial component of the drag forces produced on the filament by the gas jets attenuates the filament. Great care must be taken to control the geometry of the gas jets to provide a proper distribution in the collected filament.
One disadvantage of this system is that the angles of the gas jets require adjustment when the gas pressure or polymer flow rate is changed. Thus, careful and time-consuming control of the gas pressure and gas jet angles is required.
The molten polymer and the attenuating gas do not flow through the same nozzle. The gas jets are separated from the nozzle orifice by an insulating means such as an air space. As a result, the gas jets produce a low pressure area near the nozzle orifice which induces a flow of ambient air past the nozzle. This induced flow tends to convectively cool the nozzle and to cause the molten material to harden and obstruct the orifice: known as nozzle freeze-up.
Another disadvantage of such apparatus relates to the difficulty in controlling the spray pattern. The filament seems to wander causing an unduly broad and unfocused spray pattern. Accordingly, positioning of spray spinning nozzles for product uniformity is difficult.
In the past it has been necessary to use high throughput rates of polymeric material through the nozzle to reduce nozzle freeze-up. This produces a thicker filament, requires higher gas supply pressures to obtain higher momentum attenuating gas jets and requires the distance between the nozzle orifice and the collection surface to be greater than desired. As a result of these higher operating parameters, the nonwoven fabric produced by present spray spinning apparatus has not been entirely satisfactory. The filament is relatively thick and also includes quantities of "shot" which is solid debris or beads of non-attenuated polymer which increase cost and weight of a product and undesirably affect the feel of the nonwoven fabric. Uniformity of fiber thickness and spray pattern has been difficult to attain and maintain. Collection can be difficult and attenuation efficiency has been low. Under these conditions overall operation can be difficult.
There is a need for a nozzle attenuating system which can be continuously operated without freezing due either to conductive cooling caused by direct contact between the gas jet and the nozzle body or convective cooling caused by induced air flow when the gas jet is spaced apart from the nozzle body. A nozzle which does not readily freeze-up will permit lower polymer throughput rates and improve the resulting filament and nonwoven product. It is also desirable to have a gas jet which can be easily disassembled from the nozzle body to reduce the time necessary to clean the nozzle should it become obstructed.